As related art in automotive networking, the CAN (controller area network) protocol has been used for several years. In that case, the communication is controlled in an event-driven manner. Very great loads can be produced when the transmission of various messages is to be initiated at the same time. The non-destructive arbitration mechanism of CAN guarantees the sequential transmission of all messages according to the priority of their identifiers or identifications. For hard real-time systems, an analysis of the transit times and bus loads for the entire system is made in advance, to make sure that all message deadlines can be observed (even under peak load).
There are already communication protocols which are based on time-controlled processing, such as TTP/C or Interbus-S. The exceptional feature in this case is that the bus access is already planned in advance by allocating transmission instants. Thus, no collisions can occur during the transit time. However, in the same way, a peak load is avoided on the communications bus. Thus, the bus is frequently not completely utilized to capacity.
In the case of the TTP/C protocol, the time information is distributed by allocation of the communication objects to the global time and adjustment of the local time of the individual users by fault-tolerant averaging of all time information.
Other bus protocols derive the time information from the individual bits of a message. Consequently, a controlled run-up of the system that is as rapid as possible is not ensured in every case. In the same way, the entire system can collapse if the time-giving user malfunctions. Furthermore, automatic resumption of the function is not always ensured after the malfunction.
It is apparent that the related art is unable to deliver optimal results in every respect.